1. Field of the Invention
This invention relates to a method and apparatus for the gaseous reduction of sized or pelletized iron ores. More particularly, a direct ore reduction system in accordance with the invention involves a solid state process in which iron ore is conveyed into a shaft furnace where it is reduced at elevated temperatures by a highly reducing gaseous atmosphere. The invention has particular utility in the reduction of pelletized iron ore and sized ore particles ranging, e.g. between about 1/4 inch and 3/4 inch in diameter. For convenience the term "sized ores" will be used hereinafter to designate either beneficiated and pelletized iron ores, or ores which have been comminuted and subjected to a screening operation for separation of desired particle sizes.
The invention is equally effective for production of a partially reduced product (removal of 60% to 85% of the oxygen) for charging to a blast furnace, or a more completely reduced product (removal of 90% to 95% of the oxygen) for charging to an electric furnace for further refining in making steel.
2. Description of the Prior Art
Numerous patents and technical publications relate to gaseous reduction of iron ores either in pelletized, sized or powdered form. In general the prior art processes involve generating or reforming a reducing gas atmosphere comprising carbon monoxide and hydrogen, introducing a reducing gas atmosphere into a reduction zone containing the iron ores, subjecting the ores to gaseous reduction at elevated temperature, withdrawing the atmosphere from the reduction zone, cooling the reduced product and withdrawing it for subsequent charging to a blast furnace, or to an electric furnace.
The main components of a direct ore reduction system comprise a reducing gas generator, an ore reduction and reducing gas consumption zone, and a spent reducing gas recirculation system. In the reducing gas generator a hydrocarbonaceous fluid, ordinarily natural gas (predominantly methane), is mixed with steam and reformed catalytically into a gas mixture of high reducing capacity. The reaction proceeds by the equation: EQU CH.sub.4 + H.sub.2 O .revreaction..sub.cat. CO + 3H.sub.2 1.
in the generator the reactants also undergo the water gas shift in accordance with the well known equation: EQU CO + H.sub.2 O .revreaction. H.sub.2 + CO.sub.2 2.
the molar concentrations of the reactants, temperature and pressure conditions govern the equilibrium conditions in both equations, as is well known.
U.S. Pat. Nos. 3,160,498 issued Dec. 8, 1964 to T. F. Olt et al and 3,020,149 issued Feb. 6, 1962 to B. S. Old et al, are illustrative of prior art processes wherein a reducing gas atmosphere passes through the ore reduction stage or stages without recirculation or recycling to the reducing gas generator.
Another body of prior art involves the recycling of all or a portion of the atmosphere withdrawn from the ore reduction stage back through the reducing gas generator for regeneration with methane or other low molecular weight hydrocarbon, the reforming agent being oxygen, carbon dioxide, or water vapor. U.S. Pat. Nos. 3,375,098 and 3,375,099, issued Mar. 26, 1968 to W. E. Marshall, and U.S. Pat. No. 3,148,059, issued Sept. 8, 1964 to L. vonBogdandy, are examples of processes involving recycling of a reducing gas atmosphere for regeneration.
In the above mentioned Old et al U.S. Pat. No. 3,020,149, natural gas, air and steam are introduced into a chamber for reformation in the presence of a nickel catalyst at a temperature between about 750.degree. and 1200.degree. C. The reducing gas atmosphere leaving the chamber contains about 21% carbon monoxide, about 49% hydrogen, small amounts of carbon dioxide and water vapor, and the balance nitrogen. This reducing gas atmosphere is conducted directly from the catalyst chamber to a final reduction stage in a fluidized bed system, at a temperature not less than about 700.degree. C. The reducing gas atmosphere is then conducted to successive stages of the fluidized bed system with intermediate removal of entrained particles from the gas and reheating of the gas, and the gas withdrawn from the final reduction stage is utilized as a fuel.
In the process of the above mentioned Olt et al U.S. Pat. No. 3,160,498, steam and methane are reformed under superatmospheric pressure and at elevated temperature in the presence of a catalyst. The reducing gas mixture withdrawn from the reformer contains about 11.5% carbon monoxide, about 54% hydrogen, about 5% carbon dioxide, about 25% water, about 5% methane and balance nitrogen. The reducing gas atmosphere is then cooled in order to remove the excessive amount of water vapor therein. After cooling and condensation of moisture the reducing gas mixture contains about 15% carbon monoxide, about 71% hydrogen, about 7% carbon dioxide, about 1% water, about 6% methane and balance nitrogen. It is then necessary to reheat the gas to a temperature of about 870.degree. C before it is conducted to the reactor for reduction of the iron ore.
While the reducing gas atmospheres produced in the processes of the above mentioned U.S. Pat. Nos. 3,020,149 and 3,160,498 have a hydrogen to carbon monoxide volume ratio greater than 2:1, which is desirable because hydrogen has a more rapid rate of reduction than carbon monoxide, the reducing gas atmospheres of these prior art processes were relatively inefficient since they contained less than about 85% by volume of hydrogen plus carbon monoxide. This required throughput of a relatively high volume of reducing gas atmosphere with consequently high capital expense for pumps, compressors, conduits and associated equipment capable of handling high volume flow. Moreover, in U.S. Pat. No. 3,160,498 the high percentage of water vapor in the reducing gas atmosphere withdrawn from the catalytic reformer required the provision of a gas cooler for condensation of water vapor, followed by a preheat furnace for reheating the atmosphere back to the elevated temperature needed for the endothermic reduction or the ores. Even after removal of moisture the gas mixture contained a maximum of about 86% by volume of hydrogen plus carbon monoxide.
It is therefore a principal object of the present invention to provide a continuous, efficient process and apparatus for the gaseous reduction of iron ores wherein a reducing gas atmosphere is generated by reforming a hydrocarbonaceous fluid solely with steam in the presence of a nickel catalyst, which atmosphere contains a relatively small amount of water vapor and from about 85% to about 98% by volume of hydrogen plus carbon monoxide, thereby permitting the reducing gas atmosphere to be conducted at elevated temperature directly to the reducing zone of a shaft furnace containing sized iron ores.
It is a further object of the invention to utilize substantially all the heat values of the top gases withdrawn from the shaft furnace after reduction of the ores.